The present invention relates to a toroidal continuously variable transmission for a vehicle.
In recent years, to meet demands for increased shift comfort, improved driveability, and reduced fuel consumption and exhaust emissions, there have been proposed and developed toroidal continuously variable transmissions often abbreviated to xe2x80x9ctoroidal CVTxe2x80x9d, in which a transmission ratio is steplessly variable within limits. On such toroidal CVTs, engine power (torque) is transmitted from an input disk to an output disk via a traction oil film formed between a power roller and each of the input and output disks, using a shearing force in the traction oil film at high contact pressure. The input and output disks coaxially oppose each other. The toroidal CVT has a trunnion serving as a power roller support that rotatably supports the power roller, which is interposed between the input and output disks and is in contact with a torus surface of each of the input and output disks under preload. During transmission-ratio changing, in order to obtain a desired transmission ratio determined based on the magnitude of a gyration angle of the power roller, first of all, the power roller is shifted or displaced from a neutral position at which a rotation axis of the power roller intersects the center of rotation (rotation axis) of the input and output disks by slightly shifting the trunnion in a direction of a trunnion axis perpendicular to the rotation axis of the power roller via a hydraulic servo mechanism that operates in response to a hydraulic pressure generated by an oil pump. Usually, the oil pump is constantly driven by a prime mover (an engine) during operation of the prime mover. By virtue of a side slip force occurring in a very limited contact zone between the power roller and the input and output disks due to the slight offset (the slight vertical displacement of the power roller), the power roller is self-tilted or self-inclined. Owing to the self-inclining motion of the power roller, a first diameter of a circular-arc shaped locus drawn by movement of the very limited contact point between the power roller and the output disk on the torus surface of the output disk and a second diameter of a circular-arc shaped locus drawn by movement of the very limited contact point between the power roller and the input disk on the torus surface of the input disk, that is, a ratio of the first diameter to the second diameter can be continuously varied, thus continuously varying a transmission ratio. On the other hand, in the toroidal CVT, a degree of progress for transmission-ratio changing is fed back to the hydraulic servo mechanism, so that the trunnion gradually returns to its initial position as the transmission-ratio changing progresses. When the gyration angle based on a desired transmission ratio corresponding to a transmission-ratio command signal value has been reached, the vertical displacement of the trunnion is returned to zero, so as to terminate the inclining motion of the power roller, and to attain the return of the power roller to neutral, and thus to maintain the desired transmission ratio corresponding to the ratio command signal value.
In the prime-mover driven oil pump as discussed above, pressurized working fluid (pressurized traction oil) is continuously discharged from the oil pump during operation of the prime mover. Therefore, during the operation of the prime mover, the hydraulic servo mechanism is controllable by way of hydraulic pressure produced by the oil pump having a driven connection with the prime mover. Suppose that torque backwardly flows from road wheels to the output disk owing to hauling or coasting in a stopped state of the prime mover in which there is no hydraulic pressure produced by the oil pump for the purpose of ratio control. In this case, the hydraulic servo mechanism is in an uncontrolled state, and therefore there is an increased tendency for the toroidal CVT to undesirably shift up, for the reasons discussed below.
When the output disk is driven by road wheels due to back-flow of torque from the road wheels to the output disk, as a push-back force (a reaction force) from a contact portion between the power roller and the input shaft, the power roller, which is interposed between the input and output disks under preload, receives a component force acting in the trunnion-axis direction. This causes a slight offset of the power roller from the neutral position in the trunnion-axis direction. Owing to self-inclining motion of the power roller, an upshift of the toroidal CVT to a higher transmission ratio occurs undesirably.
If the prime mover is restarted and the vehicle is accelerated from standstill on the assumption that the toroidal CVT has been undesirably up-shifted to a high transmission ratio owing to hauling or coasting in the stopped state of the primer mover, there are the following drawbacks.
During the early stages of vehicle starting, a desired transmission ratio corresponding to a transmission-ratio command signal is generally set at a predetermined lowest ratio. Just before restarting the vehicle, there is no torque transmission, and thus the vehicle is restarted at the transmission ratio remaining high on the assumption discussed above. This is often called as a xe2x80x9chigh-ratio startingxe2x80x9d. During the so-called high-ratio starting, as a matter of course, there is a lack in torque, thus deteriorating the starting performance of the vehicle. To avoid this, Japanese Patent Provisional Publication No. 2000-9197 (corresponding to U.S. Pat. No. 6,159,126) teaches the use of a biasing device such as a return spring that biases a trunnion in one axial direction of the trunnion axis. In the toroidal CVT described in the U.S. Pat. No. 6,159,126, the spring bias of the return spring prevents an undesired offset of the power roller from its neutral position even when rotation of the road wheels in one rotational direction (either a forward-rotational direction or a reverse-rotational direction) occurs after the prime mover (the engine) has stopped. In this case, a rotational direction of the road wheels in which the spring bias acts is either of the forward-rotational direction and the reverse-rotational direction. It is desirable to prevent the so-called xe2x80x9chigh-ratio startingxe2x80x9d irrespective of whether the road wheels are rotated in the forward-rotational direction or in the reverse-rotational direction.
Accordingly, it is an object of the invention to provide a toroidal continuously variable transmission, which avoids the aforementioned disadvantages.
In order to accomplish the aforementioned and other objects of the present invention, a toroidal continuously variable transmission comprises a toroidal continuously variable transmission comprises an input disk to which rotation of a prime mover is transmitted, an output disk coaxially arranged with and opposed to the input disk, the output disk adapted to have a driving connection with and to have a driven connection with a road wheel, a power roller interposed between the input and output disks under axial preload for power transmission, a trunnion rotatably supporting the power roller to permit a tilting motion of the power roller about a trunnion axis perpendicular to a rotation axis of the power roller for ratio changing, a primary oil pump driven by the prime mover to produce a hydraulic pressure, a secondary oil pump driven in response to rotation of the road wheel to produce a hydraulic pressure, a hydraulic servo mechanism connected to the trunnion to move the trunnion in a direction of the trunnion axis so as to cause the tilting motion of the power roller by creating an offset of the power roller from a neutral position in the direction of the trunnion axis, the neutral position being a non-ratio-changing position at which the rotation axis of the power roller intersects a rotation axis of the input and output disks, a feedback device through which a degree of progress for ratio changing is fed back to the hydraulic servo mechanism so that the power roller returns to the neutral position when a desired transmission ratio has been reached, the hydraulic servo mechanism hydraulically operated by at least one of the hydraulic pressure from the primary oil pump and the hydraulic pressure from the secondary oil pump, and a hydraulic circuit that supplies the hydraulic pressure from the secondary oil pump to the hydraulic servo mechanism when the road wheel is rotated in a stopped state of the prime mover, so that an actual transmission ratio is brought closer to the desired transmission ratio.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.